Pump cups and piston seals - how to make?

[Andrewjames]

Has anyone ever attempted to make a rubber pump cup or piston seal before?

I am in the midst of researching but thought I would inquire / pick a few brains here as well..

Can something like a pump cup be produce out of rubber on a small lathe? I have most tooling for cutting metal such as carbide inserts but for rubber I am not sure if it requires special cutting tools or maybe the procedure cannot be done on a lathe?

Thanks. .a

[bezzer]

If you've got a lathe then the best method is use it to make the right size wad punch and punch out the rubber. You're wasting your time trying to machine rubber without freezing then it's still hit and miss. Working most soft materials in the lathe you need a very sharp knife tool not carbide inserts etc.

[Benelli B76]

I make them on the lathe from polyurethane. Been covered on bbs many times.

Baz

[Shed tuner]

or flat top head and an o-ring, simples

[Andrewjames]

Thanks gents,

All three ways seem possible, i was hoping to NOT have to make a wud punch to be honest but that seems like the traditional method?

if i was to make the piston from polyurethane would that indlude other thermo plastics like delrin? i guess the tollerance would have to be very tight indeed or maybe i could combine the termo plastic with a rubber oring as suggested by shed tuner..?


best
A

[Phil Russell]

I've made pump / piston washers out of leather hide before with great success. Also tried urethane but that was, to me, technically a bit more difficult. PTFE was relatively easy to turn but its tendency to retain form created an issue of the seal getting wider diameter as I shot the rifle and getting a bit 'sticky' in the cylinder so I removed it; although I have heard of people doing very well with PTFE so maybe it was my cr*p engineering. Not sure about delrin; technically you could form a seal plug but I am not certain you could guarantee a good seal because of its relative hardness. I have turned aluminium and fitted an O ring in the past which worked very well (just experimenting for fun).
Overall I have had most success, particularly with old rifles that initially had a leather seal, with making and fitting a new leather seal. I did not need a punch to do it although one could be used.
Cheers, Phil

[wil95k]

here's a guide to make leather piston seals https://www.pyramydair.com/blog/2007...r-piston-seal/

[Phil Russell]

The Pyramidair method is good but in case you do not have necessary tools here is an alternative that can be adapted as you see fit:

MAKING LEATHER PISTON WASHERS

Adapted from an article that appears on the Airgun BBS by Rob M

Material: off-cuts of sole leather from a cobbler's shop or hide from a bag maker, leather cycle saddles, leather belts, handbags. Car boot sales are good sources.

It's good to make your own washers, with a few scraps of sole-leather you have all you need to keep an old-fashioned springer working for a lifetime! It's also satisfying to make the bits you need to keep your old guns alive. These methods can help with the manufacture of leather breech seals too if you should need them.

The ways that leather washers are affixed to the piston vary from manufacturer to manufacturer. Some are held on by bolts, hex bolts or machine screws, others are riveted on.

To make the washer, measure the diameter of the old washer (or the piston) then add twice the height of the washer to the diameter. Cut out a disc this size (approximately) from your leather This can be done by:
1. Use a holesaw designed to cut holes in wood. Clamp the leather between two sheets of ply or hardboard and cut the disc. This is easy to do and gives you a centre hole from the start.
2. Cut the disc out with a sharp knife or fretsaw or similar. Find the centre of the leather disc, and drill a small hole there. Don't worry about it being a bit rough as long as you are oversize.

Optional: To size the disc, with a nut and washers, fasten the leather disc to a bolt that you can hold in the chuck of an electric drill. Now you can use the drill as a lathe, and use some clean files and wet and dry abrasive paper to round the disc off perfectly and also make the leather thinner if necessary. It has to be "Wet and Dry" type abrasive, as sandpaper or glasspaper will shed bits of abrasive that might end up embedded in the leather, this doesn't happen with "wet and dry". If using a drill or lathe wear eye protection. Depending on how much reshaping you need to do, a dust mask might also be necessary. You can shape leather like this easily by hand if you don't have power tools, but it will take a little longer.

If the leather has a shiny side, remove the shine with Wet and Dry, as it is sometimes a paint-like finish applied to the leather and could have adverse effects on the efficiency of the finished washer. Some say that the shiny side should go on the outside of the washer where it contacts the cylinder wall, but washers made of raw unshiny leather soon develop a shine after a few dozen shots anyway.

Most leather washers are actually a pair of washers; a large rear washer that is formed into a cup shape, then a smaller front washer that fits inside the cup to minimise lost volume, so you will probably end up making two discs, one smaller than the other. Sometimes the ‘inner washer’ is a countersunk steel or brass one.

Fit the washer(s) to the piston, then fasten a jubilee clip loosely around the washer to shape it into the necessary 'cup' shape. I spray or paint the washer liberally with a little aerosol of stuff I got from a cobbler's shop called 'Leather Stretcher' (isopropyl alcohol), which actually just softens leather enabling it to be formed into shape more easily. Tighten the clip gradually over a period of a few hours to get the correct shape. It's worth taking your time. This stuff really helps, and makes the job much, much easier. Tighten up the jubilee clip until the washer fits easily into the cylinder. Spray it, retighten the jubilee clip until you are happy with the fit and shape, then leave it to dry and set overnight or for a day or two if you can. If necessary, any bits of leather that you feel need removing can be carefully trimmed off now with a sharp knife.

Don't worry if, at the moment, it does not look anything like the old washer: The last step in the procedure will shape it precisely to fit the piston and the cylinder. At this point, the washer might appear much thicker than the original, and have a large gap between the back of the washer and the piston, especially around the outside, but don't worry. The important thing now is that the piston can be refitted into the cylinder without the leather washer getting damaged on threads, cocking slots, etc.

Before refitting it, let some "Bisley Gun Lubricant", or "SM50" soak into it for a few hours, or overnight, this is a silicone based oil that will not cause the leather to disintegrate over time, plus it contains molybdenum which will lubricate the leather in use. Let the Bisley Gun Lubricant soak well in, but before refitting it, you might need to dab it with a clean cloth or tissue if there is much lube on the outside of the leather. Silicone lube is good for the leather, but no good where metals will be in contact with each other, so you don't want it dripping off the washer into the cylinder or smearing the cylinder wall, for instance.

Obviously, if you are going to put GN paste or moly grease on the piston, do that now. When you have finished reassembling the gun, let the gun stand for a few days with the piston at the end of the cylinder under spring pressure (as if it has been fired), so that the spring pressure can slowly form the washer to fit the cylinder perfectly. Avoid the temptation to fire it straight away! This last step shouldn't be rushed, as allowing the leather to gradually conform exactly to the shape of the cylinder, rather than slamming it suddenly into shape by firing the gun straight away, will increase its life and help to stop it disintegrating when it gets old.

If you do all this, and haven't made the washer too big or too small, you should have an efficient and long-lasting piston head washer... It may take a short while to bed in, but it shouldn't require any more lubricant for thousands of shots now. If you should ever want to relube it I recommend taking it out of the gun to do it, rather than putting oil down the transfer port which could end up damaging the piston and the cylinder.

Cheers, Phil